Fluidized bed coating systems are known in which particles to be coated are circulated by an upwardly directed airstream through an upbed of particulate material to be coated and downwardly through an annular surrounding outbed of particulate material. Atomized coating liquid is directed centrally into the upbed for coating the particles as they travel upwardly. Airflow is generated within the coating chamber by drawing air through an upper filtering apparatus which blocks the passage of coated particulate.
It is important in such fluidized bed coating systems that the coating fluid be atomized into as small of particles as possible to facilitate efficient coating without undesirable agglomeration of liquid onto the particles to be coated. For this purpose, it is known to utilize pressurized air assisted atomizing liquid spray nozzles which facilitate fine coating liquid particle breakdown.
A problem with such fluidized bed coating systems is that the energy of the pressurized air utilized in atomizing the coating fluid tends to propel the coating particles upward past the particulate matter to be coated such as to prevent effective and efficient coating. The energy of the pressurized air further tends to propel the coated particulate into the filter system at the upper end of the coating chamber, causing clogging, operating inefficiency, and the necessity for frequent cleaning. Lowering the pressure of the atomizing air to minimize such problems, on the other end, results in ineffective atomization and liquid particle breakdown necessary for optimum coating.